A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simple network architecture. An LTE system also provides seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). Enhancements to LTE systems are considered so that they can meet or exceed International Mobile Telecommunications Advanced (IMT-Advanced) fourth generation (4G) standard. One of the key enhancements is to support bandwidth up to 100 MHz and be backwards compatible with the existing wireless network system. Carrier aggregation (CA) is introduced to improve system throughput. With carrier aggregation, the LTE-Advanced system can support peak target data rates in excess of 1 Gbps in the downlink (DL) and 500 Mbps in the uplink (UL). Such technology is attractive because it allows operators to aggregate several smaller contiguous or non-continuous component carriers (CC) to provide a larger system bandwidth, and provides backward compatibility by allowing legacy users to access the system by using one of the component carriers.
LTE/LTE-A has also defined different CA scenarios. FIG. 1 (Prior Art) illustrates CA scenarios #2, #4, and #5, where different channel characteristics are experienced for radio signals on different carriers. For CA scenario #2, component carriers are in different frequency bands (e.g., F1 and F2). F1 and F2 cells are co-located and overlaid, but F2 has smaller coverage due to larger path loss. Only F1 provides sufficient coverage and F2 is used to improve throughput. Mobility is performed based on F1 coverage. It is expected that aggregation is possible between overlaid F1 and F2 cells. For CA scenario #4, CCs are in different frequency bands (e.g., F1 and F2) and the DL transmission sites are not co-located. F1 provides macro coverage, and Remote Radio Heads (RRHs) are used to improve throughput at hot spots on F2. Mobility is performed on F1 coverage. It is expected that F2 RRH cells can be aggregated with the underlying F1 macro cells. In CA scenario #5, CCs are in different frequency bands (e.g., F1 and F2) and the DL transmission sites are not co-located. CA scenario #5 is similar to CA scenario #2, but frequency selective repeaters are deployed so that coverage is extended for one of the carrier frequencies. It is expected that F1 and F2 cells of the same base station can be aggregated where coverage overlaps.
In general, transmission on the carrier with better channel characteristics is preferred to increase system throughput and to reduce UE power consumption. In LTE/LTE-A systems, the network is responsible for both DL and UL scheduling. As a result, the network should prioritize DL/UL transmission on the carrier with better channel condition. From UE perspective, there are mechanisms that are related to carrier selection at eNB side. Those mechanisms include Channel State Information (CSI) reporting for DL channel information, Sounding Reference Signal (SRS) transmission for UL channel sounding, and power headroom report (PHR) for UE maximum transmit power configuration. From the perspective of the 3GPP specifications, there is opportunity for UE to improve UL transmission efficiency (for both spectral efficiency and power consumption) via UE maximum transmit power configuration.